Servo self-leveling mechanism



Nov. 12, 1968 w. K. BROWN 3,410,433

SERVO SELF-LEVELING MECHANISM Filed Dec. 25, 1966 66 2 INVENTOR.

WILBURN KELLY BROWN United States Patent 3,410,433 SERVO SELF-LEVELING MECHANISM Wilburn Kelly Brown, Morton Grove, Ill., assignor to Pettibone Mulliken Corporation, Chicago, 11]., a corporation of Delaware Filed Dec. 23, 1966, Ser. No. 604,393 5 Claims. (Cl. 214-763) ABSTRACT OF THE DISCLOSURE In a fork-lift truck with a pivoted boom, the fork or other loading engaging device can be automatically maintained level as the boom rises Without the usual self-leveling linkage that would be objectionable with long booms. This is accomplished by controlling the tilt cylinders, provided for other reasons, for adjusting the angularity between the load engaging device and the boom, and controlling these cylinders automatically by a servo valve. The servo valve is controlled by cables or light linkages to supply hydraulic pressure fluid to the tilt cylinders as required to maintain the load engaging device level. The servo leveling can be overridden manually. To permit this, and for safety the servo control linkage includes means which yields when the limits of valve movement are reached. The invention is illustrated in connection with a boom carried by a reaching linkage, for which it is especially beneficial. Other details are disclosed.

Introduction The invention of which this disclosure is offered for public dissemination, in the event that adequate patent protection is available, relates to self-leveling devices which are used, for example, to maintain load-lifting forks level as they are raised and lowered by a boom.

In some types of fork-lift trucks, there is no leveling problem. The forks simply ride up and down a vertical trackway or mast. However, when a boom is used for raising and lowering the forks, the pivoting of the boom for the raising and lowering operation Will change the angle of the forks carried at the end of the boom, unless some correction is made. When the correction is accomplished automatically, it is called self-leveling. If it is not accomplished automatically, it could in theory in some instances be accomplished by operator-control. Thus, in the usual situation where the angularity of the forks is controlled hydraulically, the manual valve which is used to tilt the forks when desired can also be used to correct the angularity of the forks to maintain them horizontally while they are being raised and lowered by the boom. This is not likely to be satisfactory, however, where maintaining the posture of the forks is quite important, because the operator is too unlikely to succeed in maintaining that posture well enough. Furthermore, the constant attention in an effort to do so would be very annoying and would probably result in slow speed operation. In the past, self-leveling has been accomplished by a well known type of linkage in which one or more parallelograms are provided. This well-known self-leveling linkage keeps the forks or other work handling device in a desired posture, usually horizontal. However, the linkage has to be strong enough to cooperate with the boom in supporting the load, and hence such linkage is quite heavy. If the boom is long, the self-leveling link parallel to the boom must be long, and therefore correspondingly heavy. Heavy linkage is not only an expense, but tends to decrease the lifting capacity of the machine. Not only is some of the hydraulic power available required to lift it, but if the center of gravity of the self-leveling linkage is ahead of the wheel nearest the load, it produces a tipping tendency, so that a smaller load would cause tipping, and the safe load limit is therefore lower.

According to the present invention, self-leveling is accomplished at very low cost and with very little added weight, the center of gravity of the added weight is in a position which reduces the tipping tendency. The same tilting cylinders which are needed for other reasons and which could theoretically be controlled manually to maintain the load level are automatically controlled. The automatic control is through a servo valve which is mechanically controlled by the fork device so that as the latter is swung away from the vertical in the course of raising and lowering the boom, this departure from vertical moves the valve element to supply fluid pressure to the tilt cylinders in the corrective direction, maintaining this fluid connection until the correction is complete. If the operator wishes to take control away from the servo valve, he can do so at will, by operating another valve. Thus, if he wants to tilt the forks downwardly to dump the load, he can do so. Under these conditions, the servo linkage tries to move the valve control element further than its limit movement will permit, but this tendency is made harmless by a lost motion mechanism which, while normally not yielding, will yield in this condition.

Additional objects and advantages of the invention will be apparent from the drawings and the description.

Designation of figures FIGURE 1 is a perspective view of a fork-lift truck, showing the invention applied to it;

FIGURE 2 is a detailed view of the servo valve and the spring-loaded lost motion mechanism;and

FIGURE 3 is a diagram of the hydraulic circuit of the self-leveling mechanism.

Intent clause Although the following disclosure offered for public dissemination is detailed to ensure adequacy and aid understanding, this is not intended to prejudice that purpose of a patent which is to cover each new inventive concept therein no matter how others may later disguise it by variations in form or additions or further improvements. The claims at the end hereof are intended as the chief aid toward this purpose, as it is these that meet the requirement of pointing out the parts, improvements, or combinations in which the inventive concepts are found.

Background description The invention is shown in connection with reaching and high-working fork-lift truck, but can also be used with other types. In the form shown, the load is carried by forks 11 carried by carriage 12 so that they can be moved up and down along a conventional power-telescoped mast 13. The mast 13 extends upwardly from boom 14 to which it is pivoted at 16, the angularity between the boom and mast being controlled by hydraulic cylinders 17. The boom 14 is pivoted at its rear end to an upstanding pivoted reaching link 18, pivoted at its low er end to the chassis 19 of a rough terrain type of powered vehicle, at pivot 20. The boom can be swung up and down about a pivot 21 by a pair of hydraulic cylinders 22. Each cylinder 22 is pivoted to the chassis at 24 and its piston rod 26 is pivoted to the boom at 27. The distance between the boom pivots 21 and 27 is approximately the same as the distance of chassis pivots 20 and 24. Thus, when the cylinder 22 retracts its piston 26 to provide the same length between pivots 24 and 27 as is found between pivots 20 and 21, the linkage forms a parallelogram. It is known, in fact, as a parallelogram type of linkage even though most of the time it is not strictly a parallelogram. As taught by the La Tendresse Patent No. 2,788,139, this parallelogram type of linkage provides a very desirable reaching action for the boom 14, especially when the linkage is in the condition of a parallelogram. The boom can then be retracted, or it can be advanced to reach, merely by pivoting the link 18 under control of hydraulic cylinders 29. In the parallelogram condition, this reaching action is accomplished with little or no change in the slope of boom 14.

Need for self-leveling It is quite apparent that with the cylinders 17 in a fixed condition (hydraulically locked) there is a fixed angularity between mast 13 and boom 14. It follows that if the slope of boom 14 is to change, mast 13 will no longer be vertical and its forks 11 will no longer be horizontal. In the case of forward movement, with too much variation from the vertical and horizontal conditions respectively, the load may wall from the forks. In the case of rearward movement, the load would recede from its desired position if high on the mast. Self-leveling linkage is already known in which additional linkages are added for forming parallelograms with reach link 18 and with boom 14 for automatically maintaining mast 13 vertical and forks 11 horizontal. This is disclosed, for example, in Albert Patent No. 3,001,654. There, however, the lengths are relatively short. With the long reaching link 18 and the long boom 14 illustrated, parallelogram linkage would add a great deal of weight. This additional weight is objectionable both from the standpoint of cost and from the standpoint of reducing load lifting capacity, especially in the conditions in which the weight adds a tipping force.

Servo leveling of the present invention According to the present invention, the self-leveling is accomplished with very little added weight or cost. A valve 31 accomplishes the self-leveling by supplying pres sure fluid as required to cylinders 17. The valve 31 is called a servo valve because it automatically follows the angularity of the mast and is controlled by it to perform the corrective function. The valving element of the valve, in this instance sliding spool 30 is normally controlled by rod 32, 41 which evtends to a bell crank lever having the two arms 33 and 34 which normally are rigidly connected. A point 39 on the lever arm 34 is connected by cable means 36, 36' to a point 40 on mast 13 a short distance above the pivot point 16. A single straight cable could be used, but preferably two lengths of cable connected by an idler lever 37 are used so that the cables approximately follow the contour of the boom 14. With a single cable point 40 should have the same disposition with respect to point 16 as point 39 has with respect to point 21, to provide parallelogram action. Assuming that idler lever 37 provides equal movement as to cables 36, 36', and 1 provides two parallelograms, action like that of a single parallelogram is achieved, and bell crank lever 33, 34 becomes a repeater as to the posture of mast 13.

The hydraulic connections between valves 31 and cylinder 17 are such that if bell crank lever 33, 34 were to be moved slightly in the clockwise direction as viewed, hydraulic pressure fluid would be supplied to the rear ends of cylinder 17 so as to swing the mast 13 forwardly. If bell crank lever 33, 34 is swung in the counterclockwise direction from a neutral position, valve 31 will supply hydraulic fluid to the forward ends of cylinder 17. In each instance, valve 31 connects the other end of each cylinder to the reservoir to let the hydraulic fluid out, in the typical double-acting cylinder manner. The neutral position could theoretically lock the cylinders hydraulically, but actually, as in FIG. 2, the passage is cracked open and central land 87 adjusts its clearances to apply pressure as is needed to maintain a substantially constant posture. Adjustment of the posture maintained may be by a turnbuckle 35 on cable 36 or 36.

This swinging of bell crank lever 33, 34 in one direction or the other is accomplished by cable 36, working against tension spring 38. The action occurs when the slope of boom 14 is changed. Thus, if we assume that the operator operates a valve to control hydraulic cylinder 22 to lower boom 14, this will swing bell crank lever 33, 34 in the counterclockwise direction. This follows because while cylinder 17 is locked, cables 36, 36' tie bell crank lever 33, 34 to the locked structure comprising mast 13 and boom 14, so that bell crank lever 33, 34 moves with the mast and boom. However, this counterclockwise movement of hell crank lever 33, 34, almost from its very inception, lifts the spool of valve 31 to supply hydraulic fluid to the front ends of cylinders 17 thereby moving mast 13 back towards its vertical position. As the mast 13 reaches its vertical position, cables 36, 36 will swing bell crank lever 33, 34 in the clockwise direction to move the spool of valve 31 to the neutral position again locking cylinders 17 hydraulically, or merely supplying holding pressure.

Yieldobility for overriding A valve such as 31 is normally constructed with only limited movement for its spool 30. If we assume that for some reason cylinders 17 do not make the correction indicated as boom 14 is raised or lowered, the limit of movement of the spools would soon be reached. To prevent damage to the equipment in that eventuality, some yieldable means is provided for permitting lost motion in this instance. In the illustrated form, this is provided by an assembly 41 which, because of a special purpose described below, is called an overriding assembly 41. The part of this normally visible is an outer tube 42 in which functioning parts of the assembly are housed. The interior parts include loaded spring means 43, by which the effective length of rod 32 is normally kept constant so that the spool 30 has relative movement as the shell of valve 31 moves with rod 32 normally. However, when the spool 30 reaches the limit of its relative movement in either direction, spring means 43 (when its preloading force is exceeded) permits yielding of either disk 44 or disk 45 so that bell crank lever 33, 34 can move further by elongating or shortening of the effective length of the rod 32 tube 42 combination. Nevertheless, a force is exerted to keep the spool moved to the maximum amount in the corrective direction.

Although this yielding feature is primarily a safety factor, it also provides a very simple means for cutting out the self-leveling when self-leveling is not desired. This can be done by a manually operated hydraulic valve which, when operated, causes a by-passing of valve 31, leaving control of cylinders 17 solely by the usual manual tilt valve.

Valve spool 30 is carried by rod 57, which may slide in guide 56 and is pivoted to the chassis at 58. Pivot 58 should bear the same relationship to pivot 20 that pivot 49, at the top of connecting rod 32, bears to pivot 21, when the valve is in neutral position. This makes a parallelogram with reach link 18 so that the changes of angularity of reach link 18 do not in themselves affect the valve position. Of course, if interaction between reach link 18 and cylinder 22 results in a shift of the slope of boom 14 as the reach link 18 is moved, the valve will be adjusted for correcting the corresponding departure of mast 13 from the vertical.

A suitable hydraulic circuit for this invention, including the manual override of this servo self-leveling, is shown in FIG. 3. Pump 61 is the conventional pump supplying a bank of valves of which tilt valve 62 is one. Other valves in the bank control other functions of the equipment, such as raising the boom and reaching. When valve 62 is in neutral position, the pump pressure is available to any subsequent valve in this series in the bank, as indicated by the arrow. In this condition, up line 63 is preferably hydraulically locked. Valve 62, when operated, can connect hydraulic pressure either to up line 63, leading to the forward lower ends of cylinders or down line 64, and in either event it connects the other of these two lines to tank 66.

The servo operation is powered by pump 71. When the servo mechanism is operating, pump 71 supplies oil through two-way valve 72 to servo valve 31. This valve may be of a type which in its neutral position discharges the fluid pressure to tank 66. Preferably a very slight movement from neutral makes full hydraulic fluid pressure availiable either through valve 73 to the up line 63, or to down line 64. In any event, the operation is generally as has already been described in connection with servo valve 31.

When it is desired to cut out the servo self-leveling, it is merely necessary to thrust down the handles 74 and 76, which preferably are jointly operated by a single handle 77. After this operation, valve 72 discharges fluid pressure from pump 71 back to tank 66, and valve 73 prevents discharge through the servo system from up line 63. This closing of any servo-controlled outlet from up line 63 is important so that the load will be held in a given posture or can be controlled by Valve 62.

A separate pump should be provided for the servoleveling, so that pressure fluid will always be available for that purpose. It would be within the contemplation of this invention to provide one pump for both servo-leveling and manual tilt, but for practical reasons, the separate pump for servo-leveling appears to work out less expensively. One reason is that the servo pump can be smaller than the size desired for the tilt pump, since the servo pump only needs to keep up with the angularly-slow movement of the boom.

The valve 31 may be a valve already publicly available as a servo valve, often used in steering. One type of valve found to be satisfactory is the SV-20 valve of Vickers, Inc., disclosed in that companys Bulletin M-1740-S. Some modification of the end structure for fitting into the linkage shown may be necessary. The most important consideration with respect to this valve is that with very little relative movement between the spool 30 and the cylinder 82 the conections will be shifted from the neutral position shown in FIG. 2 to supply full fluid pressure either to up line 63 or down line 64. As a matter of fact, the clearances are commercially even less than those which for the sake of clarity are shown in FIG. 2.

In the neutral position shown in FIG. 2-, the fluid pressure line 83 communicates with both of the discharge chambers 84 which are connected together by passage not shown and to discharge line 86. With a slight movement of spool 30 relatively to the right (actually movement of the valve body 82 to the left) central land 87 will close the connection from pressure pipe 83 to chamber 88, which is connected with the down port and line 64, leaving or opening the communication between the pressure conduit 83 and the chamber 89 leading to the up line 63. The same movement of spool 30 moves land 90 to close any connection between left-hand discharge chamber 84 and up chamber 89, and moves land 91 to open more fully the connection from down chamber 88 to the other discharge chamber 84.

In similar manner, the opposite direction of relative movement connects pressure line 83 to down chamber 88 and down line 64, while connecting up line 63 and its chamber 89 to the left-hand discharge chamber 84.

A chievement From the foregoing, it is apparent that with very little weight or extra equipment, automatic self-leveling of a fork device or other loading engaging means can be achieved. Of course, the same mechanism can be used for maintaining some other angularity than level, and in fact with reference to the mast, it maintains the vertical position. Although the term self-leveling may he used, regardless of the angle maintained, because it is a well understood term, posture maintenance is a more apt alternative.

Lack of stability due to unavoidable play, and deterioration of the original poor stability due to Wearing of bearing points on mechanical linkage are avoided. Loss of lifting capacity due to the weight of mechanical linkage is avoided.

By a simple valve operation, manual control to override the self-leveling feature is made effective.

Hydraulic leakage past the pistons or packing in the tilt cylinders with danger of dropping the load, is no longer a problem. The servo valve will automatically supply pressure fluid to compensate for such leakage.

Iclaim:

1. Material handling apparatus including a boom, support means for the boom comprising a pivoted reach link to which the boom is pivoted, means for raising and lowering the boom, and load engaging means carried by the free end of the boom and for which a constant posture as the boom changes slope may be desired, and double-acting tilt cylinder means for changing the angularity between the load engaging means and the boom, characterized by a servo valve having positions for, directing pressure fluid to either end of the cylinder means, and a neutral position between said positions; and light, nonload carrying connecting means controlling the servo valve in accordance with the posture of the load engaging means to operate the valve in a direction to supply pressure fluid to that end of the cylinder means which will correct any departure of the load engaging means from the desired posture; said connecting means comprising a member pivoted on the pivotal axis between the boom and its support, means cooperating with the boom to form parallelogram means coupling said member to the load engaging means at a position which has the same correlated disposition with respect to its pivotal axis such that said member is a repeater of the posture of the load engaging means, and servo linkage for operating the servo valve in accordance with the changes of posture of said member, said servo linkage being pivoted at one end to the pivoted member at a given disposition from its axis and pivoted at the other end at the same disposition from the pivotal axis of the reach link.

2. The apparatus according to claim 1, including a nullifying valve for removing the servo valve from control over the cylinder means and a manual valve for controlling supply of fluid to the cylinder means for willfully changing the posture of the load engaging means from the normally desired posture.

3. The apparatus according to claim 2, in which the connecting means and servo linkage includes normally nonyielding yieldable means for permitting greater departure of the load engaging means from the said desired posture than the departure of the servo valve from its neutral position.

4. Material handling apparatus including a boom, support means for theboom to which the boom is pivoted, means for raising and lowering the boom, and load engaging means carried by the free end of the boom and for which a constant posture as the boom changes slope may be desired, and double-acting tilt cylinder means for changing the angularity between the load engaging means and the boom, characterized by a servo valve having positions for directing pressure fluid to either end of the cylinder means, and a neutral position between said positions; and light, nonload carrying connecting means controlling the servo valve in accordance with the posture of the load engaging means to operate the valve in a direction to supply pressure fluid to that end of the cylinder means which will correct any departure of the load engaging means from the desired posture, and a nullifying valve for removing the servo valve from control over the cylinder means and a manual valve for controlling supply of fluid to the cylinder means for willfully changing the posture of the load engaging means from the normally desired posture. 5. Material handling apparatus including a boom, support means for the boom to which the boom is pivoted, means for raising and lowering the boom, and load engaging means carried by the free end of the boom and for which a constant posture as the boom changes slope may be desired, and double-acting tilt cylinder means for changing the angularity between the load engaging means and the boom, characterized by a servo valve having positions for directing pressure fluid to either end of the cylinder means, and a neutral position between said positions; and light, nonload carrying connecting means controlling the servo valve in accordance with the posture of the load engaging means to operate the valve in a direction to supply pressure fluid to that end of the cylinder means which will correct any departure of the load engaging means from the desired posture, and

a nullifying valve for removing the servo valve from control over the cylinder means and a manual valve for controlling supply of fluid to the cylinder means for willfully changing the posture of the load engaging means from the normally desired posture;

the connecting means and servo linkage, considered UNITED STATES PATENTS 2,788,139 4/1957 La Tendresse 214-141 2,811,265 10/1957 Wagner 214763 3,032,215 5/1962 French et al. 214-763 3,070,244 12/1962 Lull 214770 HUGO O. SCHULZ, Primary Examiner. 

